Minimizing growth regulators in shoot culture of an endangered plant, Hackelia venusta (Boraginaceae)

Hackelia venusta (Boraginaceae) is an endangered perennial herb endemic to the interior northwestern United States. Because of seed scarcity, micropropagation (an ex situ conservation strategy) could produce true-to-type plantlets suitable for reintroduction. We hypothesized that clones of predetermined size could be rapidly produced by supplementing multiplication and rooting media with minimal levels of cytokinin and auxin. Microshoots derived from shoot explants were cultured on Murashige and Skoog (1962) media supplemented with 1% (wt/vol) agar and 0.0001 to 10 μM benzyladenine. Inverse regression estimates on 3 genotypes predicted that a target of 2.5 axillary microshoots per explant would require a minimal level of 0.04 ± 0.02 μM benzyladenine. Culture of 25 genotypes with 0.04 μM benzyladenine resulted in an average of 2.3 ± 0.1 axillary microshoots per explant. Elongated microshoots were transferred to media supplemented with 0.1 to 25 μM indoleacetic acid. Clones rooted from 36% to 100% success after 4 wk in 2.0 μM indoleacetic acid. Plantlets transplanted ex vitro with three or more roots survived at 84% versus 46% of plantlets with fewer roots. Up to 84% of the plantlets survived in a planting trial. The data suggest that shoot culture of Hackelia venusta, with minimal growth regulators, can produce axillary microshoots for reintroduction.